The present invention relates to an exhaust gas probe.
Exhaust gas probes for motor vehicles are generally referred to as lambda probes. The function of these probes is to measure a stream of oxygen ions diffusing through a solid electrolyte layer between two measuring electrodes. ZrO2 is used as the material for such a solid electrolyte layer. A heating element in the form of a thin circuit-board conductor layer is used for heating the solid electrolyte layer to a temperature of several hundred degrees centigrade.
Constructing the entire body of the exhaust gas probe of zirconium oxide has certain disadvantages, because it leads to high leakage currents between the measuring electrodes and the circuit-board conductor layer through migration of oxygen ions of the ZrO2, whereby the service life of the circuit-board conductor and thereby that of the entire sensor are impaired. It has been proven favorable not to bring the circuit-board conductor into direct contact with the ZrO2, but rather to provide a layer in between, containing principally Al2O3, in which no migration of oxygen ions occurs.
However, producing an exhaust gas probe by sintering together layers of ZrO2 and Al2O3 presents difficulties, since the sintering temperatures as well as the shrinking rates during sintering are different for the two materials. This leads to poor repeatability of the results of the sintering process, and consequently to the possibility of producing scrap.
The differential shrinkage rates of zirconium oxide and aluminum oxide have the additional consequence that sensors having asymmetrical layer structures tend to bend, which makes incorporating them into a mount more difficult. In symmetrically constructed sensors, the different materials are under considerable pull or push stresses, which, in combination with the fluctuating temperatures to which the sensor is exposed during the course of its operational life, can result in tears in the ceramic layers and in chipping off of material.
U.S. Pat. No. 4,806,739, a plate-shaped ceramic heating element, which includes a laminate structure made of a basic substrate of ZrO2, a layer of Al2O3 applied by screen-printing technique, a circuit-board conductor layer and an exterior protective layer of Al2O3, the Al2O3 layers being tightly sintered. To prevent arching of this heating element, it is recommended that the distorting effect of an aluminum oxide layer on one side of the basic substrate be compensated so that a corresponding layer of aluminum oxide is also provided on the other side of the basic substrate. In this heating element the materials used are subject to considerable stresses.
The exhaust gas probe according to the present invention is formed by providing a controlled porosity in the layers of aluminum oxide-containing material. Thus, its elasticity is increased, and the effective material stresses in the probe may be reduced to the extent required for the mechanical stability of the exhaust gas probe.
The requisite quantity of pore-forming material may depend on the manner in which the sintering process is conducted, the coarseness and chemical composition of the layers to be sintered, and the pore-forming material used. For a given combination of these materials, however, it is possible to ascertain a suitable proportion of pore-forming material experimentally.
A densely sintered layer containing Al2O3 may be reliably and reproducibly produced when the aluminum oxide component of the material includes xcex1-Al2O3 to the extent of at least 80%.
During the operation of the exhaust gas sensor, a leakage current flows between the circuit-board conductor layer and the measuring electrodes. In ZrO2, this occurs due to the migration of oxygen ions. The tightly sintered layer of Al2O3-containing material prevents access of oxygen to the circuit-board conductor layer. A leakage current between the circuit-board conductor layer and one of the measuring electrodes can, therefore, in ZrO2, lead to an out-migration of oxygen, and consequently a blackening of the ZrO2. To avoid this result, one tries to keep the leakage current as small as possible. For this purpose, the Al2O3-containing material should contain less than 50 ppm of sodium.
Finely divided carbon may be added to the Al2O3-containing material as pore-forming material, such as in the form of glassy carbon. The compactly formed particles of glassy carbon burn during the sintering process, and, as a result, leave compact, more or less spherical pores behind. In order to obtain closed pores, pore-forming materials having an average particle size of a maximum of 10 xcexcm may be used, or an average particle size of the pore-forming material may be approximately 1 to 10 xcexcm. Furthermore, the content of pore-forming material in the Al2O3-containing layers should not exceed 12% of the solids content of these layers.
Additionally, up to 10% ZrO2 may be added to the Al2O3-containing material, for stress reduction.
In order to simplify sintering of the Al2O3-containing material, a fluxing agent may be added thereto, such as a fluxing agent containing fluorine. This may be in the form of the fluorine salt of an alkali or alkaline earth metal, such as a heavy metal such as barium, having ions that migrate in the sintered Al2O3-containing material only to a small extent, or in the form of ammonium fluoride or a fluoro-organic compound. Ammonia fluoride or a fluoro-organic compound may decompose during sintering, leaving only the flux-promoting fluorine behind in the Al2O3-containing material.